1. Field of the Invention
The present invention relates to the stabilization of polyamides, particularly against the heat degradation thereof, to the stabilizing additives utilized therefor and to the stabilized polymeric compositions thus produced.
The polyamides to be stabilized consistent herewith include, in particular: the polyamides prepared by polycondensation of saturated aliphatic dicarboxylic acids containing from 6 to 12 carbon atoms with saturated aliphatic di-primary diamines containing from 6 to 12 carbon atoms; the polyaminoacids prepared either by direct homopolycondensation of .omega.-aminoalkanoic acids containing one hydrocarbon chain containing from 4 to 12 carbon atoms, or by hydrolytic ring opening and polymerization of the lactams derived from these acids; the copolyamides prepared from the starting material monomers and comonomers of the aforesaid polyamides; and mixtures of the aforesaid or like polyamides.
Exemplary of the polyamides prepared by polycondensation of diacids and diamines, representative are, for example: nylon 66 (polymer of hexamethylenediamine and adipic acid), nylon 69 (polymer of hexamethylenediamine and azelaic acid), nylon 610 (polymer of hexamethylenediamine and sebacic acid), and nylon 612 (polymer of hexamethylenediamine and dodecanedioic acid).
Exemplary of the polyaminoacids intended, representative are: nylon 4 (polymer of 4-aminobutanoic acid or of .delta.-butyrolactam), nylon 5 (polymer of 5-aminopentanoic acid or of .delta.-valerolactam), nylon 6 (polymer of .epsilon.-caprolactam), nylon 8 (polymer of 7-aminoheptanoic acid), nylon 8 (polymer of caprylolactam), nylon 9 (polymer of 9-aminononanoic acid), nylon 10 (polymer of 10-aminodecanoic acid), nylon 11 (polymer of 11-aminoundecanoic acid), and nylon 12 (polymer of 12-aminododecanoic acid or of laurolactam).
Exemplary of the copolyamides intended, representative are, for example: nylon 66/610 (copolymer of hexamethylenediamine, adipic acid and sebacic acid), and nylon 66/6 (copolymer of hexamethylenediamine, adipic acid and caprolactam).
Those polyamides preferably and well adopted to be stabilized in accordance herewith include: nylon 66, nylon 610, nylon 6, nylon 66/610 and nylon 66/6.
2. Description of the Prior Art
The polyamides, particularly the individual nylons noted hereinabove, are of course basic materials in the plastics conversion industry. In the course of their conversion, the polyamides are typically exposed to temperatures on the order of 150.degree. C. to 300.degree. C. during their mixing and forming operations. And in use, the articles shaped therefrom are typically subjected for a fairly lengthy period of time to service temperatures on the order of 100.degree. C. to 150.degree. C. It too is known that when polyamides are subjected to the action of heat, a number of irreversible chemical and physical changes occur, the extent of which can vary as a function of the processing atmosphere (inert, oxidizing), of the processing temperature, of the duration of the process and of the state of the polymer (liquid, solid) which, in practice, manifest themselves through the appearance of the following disadvantages: the polymers become brittle, they become colored or discolored in a manner which is unacceptable for a number of applications and, in addition, their mechanical properties decline, particularly as regards tensile properties and resilience values. And it is precisely for this reason that heat stabilizers are conventionally added to polymers to prevent their degradation, or at least to reduce it considerably.
Many stabilizing systems have been proposed to this art to overcome the above-mentioned disadvantages. Many of these stabilizers give good results and are indeed in general use. Such is the case, particularly, of cuprous or cupric halides, whether used alone or in admixture with alkali metal halides; a stabilizer of this type which is particularly suitable consists of mixtures of CuI and KI.
Other types of known stabilizers consist of: aromatic hydroxy compounds such as, for example, 2,6-ditert-butyl-p-cresol, aromatic amines such as, for example, N,N'-diphenyl-p-phenylenediamine, and organic phosphorus compounds such as, for example, triphenyl phosphite.
For general outline of stabilizing systems for polyamides, reference is made, for example, to Encyclopedia of Chemical Technology, Vol. 18, pages 334 to 337 (1982) and Encyclopedia of Polymer Science and Technology, Vol. 10, page 464 (1969).
Nonetheless, despite all of the interest in the known stabilizers, research continues in this field because, depending upon the end applications for which the polyamides are intended, one stabilizing system or another always presents greater or lesser disadvantages. Thus, it is of the utmost importance that formulators have a wide variety of stabilizing systems at their disposal, from which they will be able to select those compounds which have the greatest number of technical or economic advantages for any particular application.